1. Field of the Invention
This invention relates to an infrared optical fiber used as an optical transmission line in a laser beam machine or a laser medical instrument, and it also relates to a method of manufacturing the infrared optical fiber.
2. Description of the Prior Art
In the early 1960's, a laser beam capable of forming a light spot with an extremely high power intensity was developed. The development of such a laser beam was immediately followed by the use of laser beams for processing materials at a distance therefrom. In the beginning, since the laser beam is suitable for fine processing, laser material processing was considered as a new useful technique for enhancing the miniaturization and diversification of industrial products. Then, a high-power laser beam was developed, which was used for cutting nonmetallic materials such as plywood and cloth, along curved lines. With an increase in the power of a laser beam, lasers also become used to cut or weld metal plates. Typical examples of lasers currently used for material processing include a CO.sub.2 laser and a YAG laser, both of which are used in different fields in accordance with their own characteristics.
Laser medical instruments are widely used in the field of medicine, e.g., in clinical testing, diagnosis, medical treatments, medical operations, and the like. Laser medical instruments are required to have high operativity under special conditions (good hygiene, limited space, etc.). Thus, the use of optical fibers is indispensable for laser medical instruments. The development of quartz optical fibers capable of transmitting high-power laser beams contributed to great progress in the application of lasers to the medical field.
In laser medical instruments, various kinds of lasers are currently used according to the purpose of the operation. For example, a CO.sub.2 laser is widely used in surgical operations because of its high absorptivity in the living organism and its superior incising and vaporizing capabilities. On the other hand, laser beams emitted from an excimer laser, Nd-YAG laser, or Er-YAG laser can be transmitted through a quartz optical fiber which has high flexibility. Thus, these laser beams can be directed by a quartz optical fiber into the interior of a human body for solidification and hemostasis of gastric parietes, for removing arteriosclerotic portions from coronary arteries, and for other medical treatments.
Fused quartz is used as a flexible optical transmission line because it can be processed into a fiber and also because it has an excellent transmittance for near-infrared light emitted from a YAG laser and visible light emitted from an argon laser. On the other hand, a laser beam emitted from a CO.sub.2 laser cannot be transmitted through a quartz optical fiber because it has a wavelength of 10.6 .mu.m and so falls within the category of medium infrared radiation. For the transmission of CO.sub.2 laser beams, mirror articulated optical waveguide using a combination of mirrors is employed because of its high transmission efficiency.
The mirror articulated optical waveguide, however, lacks flexibility and operativity, and is large and heavy, and has problems associated with difficulty in adjustment of its optical axis. Thus, many efforts have been made to replace it with an infrared optical fiber. As a result, some infrared optical fibers have been put into practical use for general surgery, but an infrared optical fiber having such flexibility as to be required for precise medical operations has yet to be developed.
In particular, there has recently been a need for a so-called "CO.sub.2 laser endoscope" that enables the treatment of internal organs to be performed without an external medical operation for cutting open the body, by inserting the infrared optical fiber along with the endoscope into the interior of the body to direct the CO.sub.2 laser beam to the diseased part. Furthermore, a "laser treatment within a coronary artery" has recently attracted attention, which enables the removal of arteriosclerotic part from the coronary artery without cutting open the body. But the current infrared optical fibers completely lack flexibility for these applications. The infrared optical fiber used in these applications is required to have sufficient flexibility to be easily bent within the interior of the body, and is also required to have the capability of transmitting high-power laser beams for incising and vaporizing the diseased part.
In general, when an infrared optical fiber is used for a laser endoscope, it is required to retain its power transmission capability against repeated bending of approximately 2,000 times with a curvature radius of 20 mm. When it is used for a laser treatment within an artery, it is required to retain its power transmission capability against repeated bending of approximately 100 times with a curvature radius of 10 mm. These values were obtained as follows: In fact, a laser endoscope is repeatedly used for medical operations, so that the following expression can be given; about 50 (the number of times a fiber used for the laser endoscope is bent per operation).times.40 (the number of operations in two months)=about 2,000 (the number of bending required for the fiber). In a laser treatment within an artery, an infrared optical fiber is used only for one operation. The maximum number of times the optical fiber should bend per operation was estimated to be about 100 because it should be able to pass through a tortuous path within the body.
Furthermore, an infrared optical fiber used for a laser endoscope is also required to have excellent optical characteristics such as transmittance and launching angle of the laser beam.
Metal halides such as thallium halide, cesium halide and silver halide are known as materials for an infrared optical fiber which can transmit a CO.sub.2 laser beam with high efficiency. In general, an optical fiber made of a metal halide has inferior mechanical bending characteristics and thus easily breaks. But an infrared optical fiber formed from silver halide is relatively flexible and does not easily break, so that various researches are under way for the production of an improved infrared optical fiber from silver halide.
An infrared optical fiber has been reported which is formed from a material having low water-solubility in view of stability and having a great elongation rate at rupture, i.e., capable of being greatly elongated until it ruptures, in view of flexibility, the material being prepared by adding 2 percent by weight of silver chloride to silver bromide based on the weight of silver bromide (Takahashi, et al., Sumitomo Denki (1988) No. 128, p. 123). According to the report, the obtained infrared optical fiber has excellent durability such as resistance to repeated bending (of 60,000 times with a curvature radius of 25 cm) and a long lifetime. The report has further described on an infrared optical fiber formed from materials with an improved elongation rate at rupture by adding 0.01 to 10 percent by weight of silver chloride to silver bromide or of silver bromide to silver chloride. However, while the above conventional infrared optical fibers do not break by being bent with a radius of as small as 20 mm because of their great elongation rate, they are susceptible to plastic deformation due to the bending because of their small yield stress, causing fusing at the deformed portion during the laser beam transmission, or resulting in degradation in the optical characteristics.
There has also been a report on an infrared optical fiber formed to have a diameter of 0.9 mm from a material with mechanical strength improved by using silver chloride and silver bromide in an equivalent mole ratio in its composition (A. Sa'ar, et al., Proc. Spie-Int. Soc. Opt. Eng. (USA) 843 (1988). But this optical fiber is disadvantageous in that a slight number of repetitive bending with a radius of 20 mm causes the fiber to be fused during the laser beam transmission, lowers the optical characteristics of the fiber, and sometimes causes the fiber to break.
Particularly when an infrared optical fiber is used for a laser endoscope in a medical operation, the optical fiber which directs a laser beam into the interior of the body is required to have:
1) flexibility enough to withstand the repeated bending of approximately 100 times with a curvature radius of 10 mm and the repeated bending of approximately 2,000 times with a curvature radius of 20 mm (the minimum radius with which a laser endoscope for digestive organs can bend is 20 mm); and PA1 2) excellent optical characteristics such as the capability of transmitting laser beams with an output power of 10 W or more and a small launching angle of 15 degrees or less.
For the manufacture of an infrared optical fiber, hot extrusion of metal halide materials has been employed. In a conventional hot extrusion technique, however, the infrared optical fiber has a tendency to be molded in a corrugated form as shown in FIG. 9, the extrusion speed is slow, and the obtained infrared optical fiber has a laser beam launching angle of as large as 20 to 30 degrees. This tendency becomes particularly noticeable in the extrusion of silver halide materials with improved mechanical strength, and thus it has been impossible to extrude an infrared optical fiber with excellent optical characteristics.